Broadcast is a third-party theme that is fully supported by Invisible Themes. With this in mind, I recommend reaching out to these developers directly to determine which image size should be used on this banner.
To contact Invisible Themes, I suggest following the steps listed in this help document under 'Paid themes'. Equally, you can also contact these developers directly via our Theme Store by visiting this link, clicking 'Support and documentation', and then clicking 'Get support'.
Since Invisible Themes developed this theme, they will be the best people to speak with about this to confirm the correct image size.
Best of luck with this!
I wanted to check in with you here to see if you've had a chance to speak with Invisible Themes about the banner size on your site?
If so, please let me know what size they suggested you use, as this information will likely help other Community users who are looking for the same details.